In general, LNG (liquefied natural gas) or LPG (liquefied natural gas) is employed as a liquefied gas to be used as fuel. The liquefied gas is generally contained in a cargo tank of a liquefied gas carrier, to be carried.
During carriage of the liquefied gas, a small quantity of the liquefied gas partially and naturally evaporates in a cargo tank due to an increase of the inside energy of the liquefied gas caused by movement of the liquefied gas and a temperature difference of the liquefied gas from the outside air. Such an evaporated gas is called natural boiled-off gas.
The natural boiled-off gas can be used as fuel of the liquefied gas carrier when the liquefied gas carrier is operated. However, the natural boiled-off gas is continuously accumulated in the cargo tank when the liquefied gas carrier is moored or anchored, to increase a pressure inside the cargo tank, so that various accidents such as a crack or explosion of the cargo tank occur.
Generally, the liquefied gas carrier may be largely classified, according to a propulsion mode, into two types: a liquefied gas carrier propelled by a steam turbine, and a liquefied gas carrier propelled by electric power.
FIG. 1 schematically illustrates a conventional propulsion device of a liquefied gas carrier propelled by a steam turbine. As shown in FIG. 1, the propulsion device includes: a cargo tank 100 for storing therein a liquefied natural gas; a boiled-off gas compressor 102 for compressing gas fuel supplied from the cargo tank 100; a gas blocking valve 104 for blocking supply of the gas fuel in an emergency, being connected with the Boiled-off gas compressor 102; a boiler 120 for receiving the gas fuel compressed by the boiled-off gas compressor 102 and generating steam of high temperature and high pressure; a turbine 130 driven by the steam of high temperature and high pressure supplied from the boiler 120; a propeller 190 turning by a driving force from the turbine 130; and a regular electric power generator 140 driven by using the steam of high temperature and high pressure and generating electric power. In the liquefied gas carrier propelled by the steam turbine, the propulsion device drives the turbine 130 by using the steam of high temperature and high pressure generated in the boiler 120 and generates a propulsive force by turning the propeller 190. Further, the regular electric power generator 140 is driven by using the steam of high temperature and high pressure generated in the boiler 120 to generate the regular electric power required for a ship (for example, the power for a living space in the ship).
FIG. 2 is a schematically describes a conventional propulsion device of a liquefied gas carrier propelled by electric power. In FIG. 2, a duel fuel engine 150 is mounted to the propulsion device. The duel fuel engine 150 may use any one of MDO (marine diesel oil) and gas fuel such as a natural boiled-off gas and a forced boiled-off gas, as fuel. The propulsion device with the duel fuel engine 150 of the liquefied gas carrier propelled by electric power includes: a boiled-off gas compressor 102 for compressing a natural boiled-off gas supplied from a cargo tank 100; a fuel pump 145 for forcing a liquefied gas out of the cargo tank 100; a forcing vaporizer 147 for artificially evaporating the liquefied gas from the cargo tank 100; a main electric power generator 160 for generating electric power by driving the duel fuel engine 150; a motor 170 rotatively driven by the generated electric power; a reducer 180 converting a rotating force of the motor 170 into the proper number of times of rotation and the torque; and a propeller 190 turning by the rotating force of the motor 170 and generating a propulsive force.
The aforementioned conventional liquefied gas carriers respectively propelled by the steam turbine and electric power can use the gas fuel artificially evaporated or generated during the operation as for the fuel of their respective propulsion systems. However, when the liquefied gas carriers are moored or anchored, the propulsion systems in conventional liquefied gas carriers does not need to be operated in full service condition, therefore, the accumulated fuel of gas is wasted by using a dumping device or burnt up by using a gas burner, in order to regulate the pressure inside of the cargo tank.
As mentioned above, the conventional liquefied gas carriers cause many problems, such as energy waste and economical loss thereby.